The present invention relates to a circuit for preventing a pin cushion distortion in a television receiver or a monitor having a cathode ray tube, and more particularly to a circuit which can generate a pin cushion signal having an exact period according to the frequency of a sychronizing signal.
As shown in FIG. 4, a conventional side pin cushion circuit comprises a transistor Q51 for inverting and amplifying a synchronizing signal, a transistor Q52 for generating a saw-tooth signal, transistors Q53 to Q55 with cascaded connections for outputting a pin cushion signal, resistors R51 to R67 for supplying a bias voltage to the transistors Q51 to Q55, diodes D51 to D54, and coupling capacitors C51 to C56.
The operation of the conventional side pin cushion circuit constituted as shown in FIG. 4 is described as follows.
The transistor Q51 inverts and amplifies a vertical synchronizing signal, as shown in FIG. 5A, supplied to its base through an input terminal 50 and a resistor R51 and supplies the inverted and amplified signal, as shown in FIG. 5B, to the base of the transistor Q52 through the two resistors R52 and R53. Then the transistor Q52 is turned on/off according to the logic state of the inverted and amplified vertical synchronizing signal supplied to its base, to open and close the current passage connected through its collector and emitter to a second power source GND. At this time, the capacitor C51 charges and discharges according to the switching operation of the transistor Q52, thereby generating saw-tooth signal as shown in FIG. 5C to supply to the base of the transistor Q53.
Meanwhile, the transistor Q53 with the resistor R56 and the capacitor C52 integrates and amplifies the saw-tooth signal supplied to its base and generates a pin cushion signal such as that of FIG. 5D. And the transistor Q53 supplies the generated pin cushion signal to the base of the transistor Q54 through a coupling capacitor C53. At this time, the resistor R55 limits the current flowing in the capacitors C51 and C52.
The transistor Q54 together with four resistors R57 to R60 constitute an amplifying circuit, which inverts and amplifies the pin cushion signal supplied to its base into the pin cushion signal of FIG. 5E and supplies this inverted and amplified pin cushion signal to the base of the transistor Q55 through a current limiting resistor R61, a coupling capacitor C54, and a current limiting resistor R62.
The transistor Q55 together with four resistors R63 to R66 and two capacitors C55 and C56 constitute another amplifying circuit, which again inverts and amplifies the inverted pin cushion signal of FIG. 5E supplied to its base, and supplies an amplified pin cushion signal such as that of FIG. 5F to a mixer (not shown) through a resistor R67 and an output terminal 51. Then the mixer amplitude-modulates the horizontal deflection signal of a saw-tooth waveform supplied from a horizontal deflection portion (not shown) according to the pin cushion signal, and supplies the modulated horizontal deflection signal to a deflection coil of a cathode ray tube.
However, the side pin cushion circuit, as shown in FIG. 4, generates a distortion of the signals in the steps of amplifying the vertical synchronizing signal of the square wave form, converting the amplified vertical synchronizing signal to a saw-tooth signal, and again converting the converted saw-tooth signal to a pin cushion signal, such that it cannot exactly keep the period of the pin cushion signal. Because of this, the conventional side pin cushion circuit is applicable only to a cathode ray tube displayer for a single mode having a constant frequency of the vertical synchronizing signal. It is not applicable to a cathode ray tube displayer for multiple modes using a plurality of vertical synchronizing signals having different frequencies, because the distortion of the pin cushion signal according to the frequency of the vertical synchronizing signal is generated.